Several of them have finally been completed.  A model was constructed several months ago to test the theory of these anti-gravity vehicles and it worked.  A few months ago I constructed a larger version to see if a hovercraft would support human weight. That also worked and so I am now in the process of constructing a larger craft with a propulsion and steering system.  You can now monitor the progress of this craft. Keep watching in the near future for further advancements. 

What's a hovercraft? 
A hovercraft is an all-terrain (Land, Water, Snow...) vehicle capable a floating over and clearing objects on the ground.
This question I have heard far too many times.  I will attempt to explain how they work.
Suppose you had a flat piece of wood and it was at an elevated position. What would happened if it was to be dropped? Well of course all the air under it would move outward.  Now suppose that the air being forced outward hit a wall of some sort.  It would be forced inward again.  This is how a hovercraft works.  Air is thrusted through the hull at some point and that air is forced to the outside of the craft because of the weight on top of it.  When the air hits this wall, it is forced back into the center to create a "cushion" of air to ride on.  Then, with the craft being nearly frictionless, it takes very little to give it a push and send it forward.  So a relatively small fan will do the job.

I have only built two hovercrafts so far.  The first  is only a model.  I built it in about an hour just for fun and out of curiosity about whether it would actually float.  I had everything to build it in my own home.  The next hovercraft (Hover II) is actually capable of lifting a human. This craft took a bit more effort and time.  A friend and I built it in an afternoon.  My current project is more like a summer project.  It is much larger than the other two crafts and will have a propulsion system.  You can watch its progress on this page.

• Hovercraft I - small model hovercraft

• Hovercraft II - larger scale of model -This craft has just recently been constructed and supports human weight.

• Hovercraft III - larger dimensions/propulsion system - This craft is currently under construction.  On this page you can monitor the craft's progress.

• Hovercraft IV - look at www.hovercraft.com